The present invention relates to a vehicle communication system that is mounted in a vehicle to be used for communications with other vehicles, and also relates to a technique useful in application to, for example, prevention of vehicle collision.
For the purpose of safe-driving support and efficiency of road traffic, an intelligent transport system (ITS) has been widely spread. As one of safe-driving system services, there has been studied a service in which vehicle communication systems mounted in running vehicles which are adjacent to each other perform wireless communications with each other to provide running states of the host vehicles, so that their drivers receive a warning of avoidance of collision accident in traffic. In an inter-vehicle communication network comprised of such vehicle communication systems, communications are performed by autonomously forming network without base stations, unlike a network system that is used for mobile phones and is controlled by base stations. In the inter-vehicle communication network, the vehicle communication system of each vehicle exchanges vehicle running information showing a running state of each vehicle such as the position, velocity, and moving direction of each host vehicle with those of the other vehicles, together with the IDs of the respective vehicles. The vehicle running information is converted into a packet that is a set of information to be transmitted from an antenna as a transmission signal.
A reachable range of the transmission signal corresponds to a communication range of the vehicle communication system. Transmission to all vehicles in the transmission range by the vehicle communication system is called as broadcasting. Since the broadcasting is performed using only the same wireless channel, the running state can be received from the other vehicles in the communication range among the adjacent vehicles in the communication range, and a safe-driving support service is provided using the information.
When performing the inter-vehicle communications, a wireless channel is shared among a plurality of vehicle communication systems. Thus, only one vehicle communication system can transmit information in the transmission range, and the other vehicle communication systems can only receive the information. Therefore, when a plurality of communications systems perform communications at the same time, a signal collision phenomenon that is called as wireless collision occurs, resulting in failure of communications due to damage of a packet during transmission.
For the purpose of providing a safe-driving support service, the inter-vehicle communications are performed. Accordingly, it is important to perform communications without any trouble even when a plurality of vehicles are located in a communication range. The communication range of a host vehicle is herein defined as a host-vehicle network. The maximum capacity number N of vehicles which can be accommodated in the host-vehicle network is obtained as follows.
In the first place, when a transmission time Ts of data is obtained, a packet transmission time Ts0=Dx/Vx can be obtained from a transmission period Td of data, a transmission velocity Vx of data, and a data amount Dx communicated in a transmission range. Further, in a wireless collision avoidance technique called CSMA/CA (Carrier-Sense Multiple Access with Collision Avoidance) in which when starting communications, data are transmitted after conforming that other communication systems do not perform communications, a sum Ts1 of a time required for confirming that the wireless channel is not used and a time required for switching transmission and reception is obtained. The transmission time of data is obtained as Ts=Ts0+Ts1. Accordingly, the maximum number of vehicles which can be accommodated in the host-vehicle network within the communication range is obtained as N=Td/Ts. For example, if Ts0=0.01024 ms and Ts1=1 ms obtained from Td=50 ms, Dx=1024 bits (packet volume of 128×8 bits), and Vx=1 Mbps, the maximum capacity number N of vehicles is 49.
For example, when a vehicle is stuck in traffic, there are located vehicles the number of which is larger than the maximum capacity number, resulting in difficulty of inter-vehicle communications in the communication range of the network.
Therefore, it has been studied that wireless collision is reduced by changing a transmission power and a transmission period. The transmission power herein means an output voltage value set in an inter-vehicle wireless system. The transmission period is a transmission period of a packet transmitted by a vehicle communication system.
A long transmission period of packets reduces the transmission frequency of packets in each communication system, and thus reduction of wireless collision can be advantageously expected. Further, when the transmission power is reduced, the communication range is narrowed and the number of vehicles in the communication range is reduced. Thus, reduction of wireless collision can be advantageously expected. However, if the transmission period is too long for the purpose of safe-driving support such as collision avoidance, there is a problem that the vehicle running information necessary for collision prevention does not reach. For example, in comparison between transmission periods of 1 second and 0.1 second in which the vehicle running information is transmitted, a vehicle running at a velocity of 60 km/h advances 16.6 m per second, and 1.6 m per 0.1 second. Thus, vehicle positional information that can be periodically obtained in accordance with a predetermined period can be obtained every 16.6 m and 1.6 m, which makes a significant difference in accuracy to avoid collision by obtaining the positions of the other vehicles. Further, if the transmission range is needlessly narrowed, a communication range necessary for traffic safety support cannot be secured, and the vehicle running information of other vehicles located out of a range where a collision accident avoidance service in traffic can be provided does not reach.
For example, in Japanese Patent Laid-Open No. 2008-245268, a wireless communication device mounted in a vehicle incorporates therein a microcomputer to which a running velocity V is input in order to address the problems of the transmission power and the transmission period. The microcomputer calculates a transmission period Td that becomes shorter as the running velocity V becomes higher, and a communication output that becomes larger as the running velocity becomes higher, and transmits information relating to the vehicle such as a running state at the transmission period Td. Therefore, necessary communications can be secured and communication traffic can be reduced.